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Infrared and nuclear magnetic resonance studies on metal complexes of 3-substituted 2,4-pentanediones
133 Contribution No. 1.36 from the Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Fukuoka, Japan
Infrared and Nuclear Magnetic Resonance Studies on Metal Complexes of 3-Substituted
2,+Pentanediones
Yukito Murakami,* Katsuyuki Nakamura, Hiroaki Uchida, and Yukiko Kanaoka Received
April 27,
In an effort to understand the nature of chelate rings of 2,4-pentanedione complexes, infrared and nmr spectra of some divalent and trivalent metal complexes of 3-substituted 2,4_pentanediones, where phenyl, pbromophenyl, p-cyanophenyl, p-nitrophenyl, benzyl and p-nitrobenzyl were these substituents, have been investigated. The introduction of any of these substituents to the 2,4-pentanedione complexes resulted in the lower frequency shift of C=O stretching vibration and the higher frequency shift of M-O stretching, Thus, these substituents tend to increase the strength of coordinate bond, and the relative strength of -M-O bonds follows the sequence: Cut’ < Pd”; Fe”r < Crr” < Cont. Three structural formula have been put forward to elucidate the nature of chelate rings of various metal complexes. The relatively minor effect of a phenyl group, which is comparable with a benzyl group, is probably due to the twist of a phenyl ring from the plane of a chelate ring. Nmr spectra have not provided much useful information.
1968
infrared study has been extended to the far infrared region, where the metal-ligand stretching vibration is expected to appear, and the normal coordinate analysis ‘JO has been shown even more efficient in cooperation with deuterium9,” and oxygen (**O)” isotope techniques. In connection with the previous study on the electronic spectra of some trivalent metal complexes of 3-phenyl- and 3-benzyl-2,4-pentanedione,12 the present investigation deals with the infrared spectra of metal complexes of 2,4-pentanediones (I), which carry phenyl,
R = GH.T,
p-BGHT,
p-CNGHi;
pNOGHt,
CJ-LCHF,
p-NO,CbH,CHr Introduction
M = Ben, Cuit, Pd”; Al”‘, CrI*r, Fe”‘, Co’11
In recent years, the infrared spectral method has been much applied to the metal chelate systems of 2,4pentanediones for the elucidation of the nature of The these chelate rings and coordinate bonds.‘-’ normal coordinate treatment of these metal chelates has been much successful in the analysis of their vibrational spectra and become reliable basis for the In adassignment of those characteristic bands.‘,’ dition, the isotope techniques, where 2,4-pentanedione complexes labeled with deuterium, oxygen (“0) and carbon (13C) isotopes were used, have provided much valuable information for the assignments of absorption bands in the NaCl region and for the valence-tautomerism involved in the chelate rings.7,8 Meanwhile, the
p-substituted phenyl, benzyl or p-substituted benzyl group at the 3-position, in the 4000-200 cm-’ region. The assignments of vibrational bands have been performed by referring to the results of normal coordinate calculation on the 2,4-pentanedione complexes and to some empirical data. ‘J*‘~ The nmr measurements have also been added to understand the nature of these chelate rings.
Experimental Section Preparation of Metal Complexes.
procedures (*) To whom all the correspondence should’ be addressed. (1) K. Nakamoto and A. E. Martell, 1. Chem. Phys., 32, 588 (1960). (2) K. Nakamoto, P. 1. McCarthy, A. Ruby, and A. E. Martell, I. Am. Chem. Sot., 83; 1066 (1961). (3) K. Nakamoto, P. 1. McCarthy, and A. E. Martell, I. Am. Chem. Sot., 83, 1272 (1961). (4) K. Nakamoto. Y. Morimoto, and A. E. Martell. I. Phys. Chem.. 66, 346 (1962). (5) K. E. Lawson, Specfrochim. Acta, 17, 248 (1961). (6) R. D. Gillard, H. G. Silver, and I. L. Wood, Spectrochim. Ado, 20. 63 (1964). (7) H. Musso and H. lunge, J‘elrahedron L&f., 4003 (1966). (8) H. Musso and H. Junge, Chem. Ber., 101, 801 (1968).
The preparative for the divalent and trivalent metal com-
(9) G. T. Behnke and K. Naknmoto, Jnorg. Chem.. 6, 433 (1967). (IO) M. Mikami, 1. Nakagawa, and T. Shimanouchi. Specfrochim. Acfu, 23A. 1037 (1967). (1 I) S. Pinchas. B. L. Silufz, and I. Laulicht, 1. Chem. Phys.. 46. 1506 (1967). (12) Y. Murakami and K. Nakamura. Buff. Chcm. Sot. lapan, 39, 901 (1966). (13) See. e.g., N. B. Colthup, L. H. Daly. and S. E. Wiberley. .Introduction to Infrared and Raman Spectroscopy*, Academic Press, New York (1964).
Murakami, Nakamura, Uchida, Kanaoka 1 Infrared and Nuclear Resonance Studies on Metal Complexes
134 Table I. Analyses of the metal complexes of 3-substituted 2,4- pentanediones 3-Substituent
C
Complex Pound
Calcd.
Analyses, % H Calcd. Pound
N Found -
Calcd. -
Phenyl
73.21 63.63 58.18 71.40 68.59 67.74 66.03
73.52 63.83 57.85 71.73 68.62 68.17 67.81
6.32 5.49 5.08 6.21 5.78 5.85 5.68
6.17 5.36 4.85 6.02 5.76 5.72 5.69
p-Bromophenyl
51.26 46.54 48.42 48.23 48.01
51.09 46.22 48.67 48.45 48.26
4.02 3.81 3.76 3.98 3.85
3.90 3.53 3.71 3.70 3.68
-
-
pcyanophenyl
70.21 62.13 65.88
70.40 62.13 65.86
5.37 4.36 5.02
4.92 4.35 4.61
6.84 6.41 6.06
6.84 6.04 6.40
p-Nitrophenyl
60.82 52.19 56.24 55.00
58.79 52.43 55.32 55.08
4.73 4.17 4.55 4.41
4.49 4.00 4.22 4.20
5.90 5.83 4.76 5.64
6.23 5.56 4.86 5.84
Benzyl
74.10 65.19 59.50 72.61 70.06 69.38 68.74
74.40 65.22 59.45 72.71 69.78 69.34 69.00
6.79 6.13 5.65 6.75 6.35 6.58 6.44
6.76 5.93 5.41 6.61 6.34 6.30 6.27
-
-
p_Nitrobenzyl
60.34 53.97 57.48 60.37
60.37 54.19 57.29 57.00
5.29 4.69 4.86 4.95
5.06 4.55 4.81 4.78
5.92 5.19 5.91 4.97
5.87 5.27 5.57 5.54
plexes of 3-substituted 2,4_pentanediones were essentially similar to those employed for the corresponding 2,4-pentanedione complexes. A part of these experimental results have been described previously.14 The results of microanalyses” are listed in Table I. Spectral Measurements. Infrared spectra in the region 4000-650 cm-’ were recorded on a Koken Model DS-30 1 spectrometer equipped with sodium chloride optics. The KBr disk method was employed for the most complexes in this region, whereas the nujol mull technique was adopted for the metal complexes of 3-(p-nitrobenzyl)2,4-pentanedione. A Hitachi Model EPI-L grating spectrophotometer was used to obtain the spectra in the 700-200 cm-’ region. For these spectra, all the samples were measured as nujol mulls sandwiched between polyethylene film. Calibration of frequency readings was made with polystyrene film, and water vapor as well as carbon dioxide in the air. Nmr spectra were recorded on a Varian A-60 spectrometer. Chemical shifts are reported in ppm from internal tetramethylsilane (TMS) and data are good to 0.5 cps. The spectra were calibrated by the use of chloroform signal (436 cps from TMS) as the secondary reference.
(14) K. Nakamura and Y. Murakami, Tech. (Kyushu Daigaku Kogaku Shuho), 38, 329 (1965). (15) The analyses of all the metal complexes Microanalysis Center of Kyushu University.
Inorganica Chimica Acta I 2:2
i june,
Reports were
1968
Kyushu Univ.
performed
at the
Results and Discussion Infrared Spectra. The region 4000 to 2250 cm-’ has been excluded from this discussion, since no significant absorptions are seen except the C-H stretching bands. Then, the infrared frequency range treated in this work is divided into six regions for the sake of discussion: 2250-1300 cm-‘, 1300-1050 cm-‘, 1050-1000 -* 950-900 cm-‘, 900-700 cm-’ and 700-200 cm-‘. Fte ‘characteristic absorption bands along with the most plausible assignments for the 3-phenyl-2,4pentanedione complexes are summarized in Table II, while absorption peaks assigned to the C=O stretching and the M-O stretching modes for all the metal complexes treated in this work are listed in Tables III and IV, respectively. The observed bands for the metal complexes of 3-phenyl-2,4-pentanedione are shown schematically in Figure 1 in the 1600-700 cm-’ region for the sake of correlation. Figures 2 and 3 illustrate the far infrared spectra for the same metal complexes in the 700-200 cm-’ region. The 2250-1300 cm-’ region involves several distinct bands which are assigned to the C-O stretching, the C=C stretching, the CHs deformation and the C-CBS stretching modes in addition to the benzene-ring stretching vibrations. Although the assignment of vibrational bands for the 2,4-pentanedione complexes has been made on the purely empirical basis,r6,i7 (16) 1. Lecomte. Discussions Faraday Sot., 9, 125 (1950). (17) R. Mecke and E. Funck, Z. Elekfrochem., 60, 1124 (1956).
135 Table II.
Infrared
absorption
bands of the metal complexes
of 3-phenyl-2,4-pentanedione
Ben
Cu”
Pd”
Al”1
CrIn
Fc’ll
Co”
1582 vs 1459 s
1576 vs 1490 s 1454 s
1554 vs 1490 w
1595 vs 1462 s
I575 vs 1490 w 1453 m
1573 vs 1490 w 1452 s
1439 sh
1424 s
1439 m
1439 m
1437 s
1383 s 1369 s 1335 m 1250 w 1179vw 1157 VW 1073 w 1021 s
1381 s 1381 s 1315 s 1175w 1154w 1075 w
1428 m 1420 m 1360 m 1335 s 1304 m 1241 w 1176~ 1155w 1072 w 1036 w 1014 m 1003 w 976 w 929 w
1570 vs 1490 w 1477 w 1451 m 1423m t
1396 s 1372 s 1330 m 1155vw 1075 w
1373 m 1349 s 1318 m 1178~~ 1157vw 1074 w
1363 s 1346 s 1318 s 1173 w 1158~ 1074 w
1038 w 1019 m
1036 w 1014 m
1035 w 1012 m
983 w 932 m 749 m 705 m 1042 s 856 m 845 s 684 w 660 s 617~ 553 s
499 w 462 w 449s 440s 430 w 369 w 283 w 245 m, br s = strong,
Table
Ill.
1035 w 1012 m 979 930 921 764 710
w w w s s
768 w 721 w 706 w
983 w 930 w
979 w 927 m
770 w 707 m
778 sh 766 w 709 m
970 926 916 769 764 707
Assignments
benzene
693 s 618w 556 m
683 m 665 w 625 m 608 s 560 vw 533 m
684 661 634 617 538
470 457 426 393
474 458 442 408
508 492 455 448 432 425
466 s, br 439 s 397 m
s s m s
305 w 271 m
313 m 227 s
m = medium,
The C=O
s m w w
bands
350 s 291 w 247 w
377 m 313w 239 w
and w = weak intensity;
stretching
s s m m sh m
(cm-‘)
m VW s s w
v = very,
sh = shoulder,
for the metal complexes
ring
1360 m 1349 m 1315 m
C=O str CH, sym def C-C + C-CH,
1155 w 1074 w 1225 w 1 1037 w
benzene in-plane
str
C-H
CH, deg rock
1018 1003 w 1 980 w 929 w \ I 765 w
..... C-CH,
+ C-O
ring def + M-O C-CHI bend +M-0 str
680 w 655 w 618w 598 s 588 s 557 VW 522 w 451 s 437 s 425 s 415 sh 390 m
688 s 650 s 617~ 555 m
473 s 421 w
M-O
str
315 s 234 m, br 220 sh
385 w 361 m
M-O
str
of 3-substituted
2,4-pentanediones
Ligand
Ben
cun
Pd”
Al’”
Cr”’
Fe”’
Co”’
1582 1583 1582 1582 1578 1585
1576 1574 1568 1575 1569 1571
1554 -
1595 1595 -
1575 1573 1568 1583
1573 1570 1571 1570 1570 1566
1570 1570
Abbreviations are made for ligands: PAA, 3-phenyl-2,4-pentanedione; NPAA, 3-(p-nitrophenyl)-2,4-pentanedione; cyanophenyl)-2,4-pentanedione; benzyl)-2,4_pentanedione.
Nakamoto et al.‘~*first applied the normal coordinate treatment to these metal complexes and the strong band at higher frequency side in the 1600-l 500 cm-’ region was assigned to the C=C stretching mode. Recently, however, Behnke and Nakamoto has made the normal coordinate analysis of dichloro(2,4-pentanediono)platinate( II) by introducing a modified Urey-Bradley force field? In cooperation with the deuterium isotope effect on vibrational spectra of the 2,4-pentanedioneplatinum complexes, they have proposed to interchange the band assignments for the C-0 and the CzC Murakami,
Nakamura,
Vchida,
Kanaoka
str
and br = broad.
PAA BrPAA CPAA NPAA BAA NBAA
1556 -
str
benzene C-H out-of-plane
704 m ) I I
682 m 664 w 635 s 618 w 541 m
str
CH, deg def
I
w w m sh w m
(+C-C)
C=O
1570 1567 -
BrPAA, 3-(p-bromophenyl)-2,4-pentanedione; CPAA, 3-(pBAA, 3-benzyl-2,4-pentanedione; NBAA, 3-(p-nitro-
stretching mode. This result has been supported by investigation of the vibrational band-shift in the infrared spectra of the ‘80-labeled 2-4-pentanedione complexes of chromium( I I I) and manganese( I II),” and also of the *H-, “0-, and “C-labeled 2,4-pentanedione complexes of copper( I I)? Another interchange of the band assignments made by Nakamoto and his coworker’ is for the CH3 degenerate deformation and the lower frequency C-0 stretching mode, and this has been supported by the infrared study of the corresponding siotope-labeled complexes as reported by Musso and 1 Infrared
and Nuclear
Resonance
Studies
on Metal
Complexes
136 Table
IV.
The M-O
Ligand
stretching
Be”
PAA
1042 856 845
BrPAA
1034 844 482 1037 853 489 1031 855 505 859 508 1015 863 507
bands
(cm-‘) for the metal complexes
CU”
Pd”
of 3substituted
Al”’
2,4-pentanediones
Cr”*
Fetn
Co”’
470 313
474 305
508
466 350
451 315
473 385
463 298
-
-
470 355
453 312
478 386
467 314
-
-
-
452
468 313
-
-
-
453 311
474 384
465 292 465 310
478 297 -
505
469 372 467 363
452 318 456 317
479 391 -
499
CPAA NPAA BAA NBAA
Abbreviations
for ligands
are decribed
-
312
-
in Table III.
Ben Cu” Pd”
Crrn Feru
Figure 1. complexes
Observed vibrational frequencies of various metal of 3-phenyl-2,4-pentanedione in the 1600-700 cm- ’
.-:
700
600 WAVE
&I
NJMBER
&I ’ &I ’
I 200
ICH-‘)
Figure 3. Far infrared spectra of trivalent metal complexes 3-phenyI2,4_pentanedione (nujol mull method).
WAVE NlM8ER
KM-‘)
Figure 2. Far infrared spectra of divalent metal complexes of .3-phenyI2,Cpentanedione (nujol mull method). Inorganica
Chimica
Acta
1 2:2
1 lune, 1968
of
Junge.’ The two strong bands in the 1600-1500 cm-’ region observed for 2,4pentanedione complexes seem to be merged into one strong band in the present chelate ,,“.A +l.:* &,,..A :- (133LUUSU ,.^,...... ,.A e,. ,..:-,..:1.. ,I.., n.rn*--0 Llll” 3J3LG1113, L‘LLD “a,lU ID L” L, “F; ~“““cl”‘y uue to the Cc0 stretching vibration. When 3-(p-nitrophenyl)-2,4-pentanedione and 3-(p-nitrobenzyl)2,4pentanedione are used as ligands, the asymmetric and the symmetric NO* vibrational mode appear evidently at around 1515 cm-’ and 1350 cm-‘, respectively. On the other hand, in the case that 3-(p-cyanophenyl)-2,4pentanedione is a ligand molecule, the band near 2220 cm-’ is due to the CN stretching mode.
137
In the regions 1300-1050 cm-’ and 900-700 cm-‘, there are observed several absorption bands which are characr=rist:c of benzene C-H in-plane bending and out-of-l lane wagging vibrations, respectively. In addition to these vibrational modes, the aromatic C-Br stretching mode in the 3-(p-bromophenyl)-2,Cpentanedione complexes is expected to appear in the former region although the definite position is not certain in the present work. On the other hand, the latter region involves the C-N stretching mode for the nitrocompouds; i.e., in the 854-860 cm-’ range. The CH, rocking mode in the 1050-1000 cm-’ region resulted in a larger number of bands in most cases by introducing a substituent at the 3-position of 2,4pentanedione. In the lowest frequency region in this work, 700-200 cm-‘, the vibrational spectra of all the metal complexes became much more complicated than the corresponding 2,4-pentanedione complexes, due to .the presence of an extra substituent. However, a strong band in the 450500 cm-’ range and another band of various intensity in the 300-400 cm-’ range are assigned to the metaloxygen stretching coupled with other vibrational modes for the copper, palladium, chromium, iron and cobalt complexes. Behnke and Nakamotog has assigned the lower frequency band near 300 cm-’ to the C-CHJ bending mode in their study of the 2,4-pentanedioneplatinum(I1) complexes. On the other hand, Mikami, Nakagawa and Shimanouchi*” have observed that the lower frequency band is most metal-sensitive. The latter assignment is preferred in the present work as described above. The spectra of all the beryllium complexes are markedly different from those of the corresponding complexes of other metals in three regions; 1050-1000 cm-‘, 900-700 cm-’ and 700-200 cm-‘. In conformity with the previous assignments made by Nakamoto et aL3 for the 2,4-pentanedione complex, the strong bands appearing near 1030 cm-’ and in 860-830 cm-‘, and the medium band near 500 cm-’ are assigned to the Be-O stretching mode coupled slightly with other vibrational components. It is reasonable to expect that the beryllium complexes demonstrate their metalsensitive bands in relatively higher frequency region than other metal complexes, due to the light mass of beryllium atom and the very short Be-O distance. Judging from the relative positions of the C-O bands in the 1600-1500 cm-’ range among the metal complexes of each 3-substituted 2,4-pentanedione, delocalization of ligand z-electrons in a chelate ring tends to increase in the_following sequence: for divalent
metals,
Be < Cu< Pd
for trivalent metals, Al< Cr < Fe <, Co. complex demonstrates the C=O vibrational mode at the lowest frequency in the above range
The palladium among ligand, among
the metal complexes of each corresponding The band-shift of this mode is seen to be minor other transition metal complexes. The C--O bands in the beryllium and aluminum complexes are observed at relatively higher frequency than the other metal complexes as shown in Table 111 and the delocalization of n-electrons in a chelate ring of the Murakami,
Nakamura,
Uchida,
Konaoka
beryllium co.nplexes seems in general to be slightly greater than the corresponding aluminum complexes. Another C-O stretching mode appearing in the 14001360 cm-’ range varies in a manner parallel to the higher frequency mode among the metal complexes of each 3substituted 2,4-pentanedione. The metal-oxygen stretching modes appearing in the 700-200 cm-’ region may directly provide the information on the relative strength of coordinate bonds, and the force constant of the oxygen-metal bonds in the transition metal complexes of each ligand species follows the sequence: for divalent metals, Cu< Pd for trivalent
metals,
Fe
This is in agreement with the results obtained by the two research groups on the 2,4-pentanedione complexes. 2,3.‘oThe coordinate bond of a palladium complex seems to be strongest among those of the transition metal complexes treated in this work. In order to elucidate the electronic structure of various metal complexes investigated in this work in conformity with the electronic spectral dataI as well as the present results, three structural forms are put forward below: i.e., strong (II), weakly g-bonded (III) and strongly a-bonded (IV) chelate rings. In strong
chelates, ligand n-electrons as well as d-electrons of the metal tend to be well delocalized in a whole chelate ring. The palladium chelates are typical of this structure. In weakly c-bonded chelates, ligand n-electrons are well extended to the coordinate bonds and tend) to be delocalized in a whole chelate ring. Nevertheless, weak a-bonding provides relatively weaker coordinate bonds as a whole. This seems to be the case for the iron complexes. As understood by investigating the X+X* transitionI and the carbonyl stretching mode for the first transition metal complexes of 3-substituted 2,4pentanediones, x-electrons are well delocalized in a chelate ring.and d-orbitals of a trivalent metal atom are involved in such a x-system. This n-electronic interaction in the iron complexes is relatively large among the trivalent metal complexes of present concern and greater than that in the corresponding chromium complexes. Nevertheless, the coordinate bonds involved in these iron complexes are weaker than those in the corresponding chromium complexes as the M-O stretching vibrations appearing in the far infrared The above inconsistency can be region indicate. ascribed to the weaker c-bond character of the coMeanwhile, ordinate bonds in the iron complexes. ligand x-clcctrons in the beryllium and aluminum complexes most likely tend to be delocalized only in a ligand part as judged from the C=O vibrational modes , Injrared
and Nucleur
Kesonunce
Studies
on Me&d Complexes
138 and the electronic 7c+rc* transitions” of these metal The third structural form (IV) may be complexes. assigned to these non-transition metal complexes. The introduction of either phenyl or benzyl derivative at the 3-position of 2,4-pentanedione resulted in the lower frequency shift of C-O stretching vibration and the higher frequency shift of M-O stretching vibration. Thus, the substituents employed in this work are in favor of forming a stronger chelate ring. However, the characteristic bands cited here do not vary significantly among the various 3-substituted 2,4pentanedione complexes of each metal species. The variation of far infrared spectra due to the change of ligand is shown in Figure 4 for the iron complexes. Thus, the effect of the substitution with a phenyl group on the electronic structure of a chelate ring is almost the same as that with a benzyl group; in addition, psubstituents of these groups do not cause much change on the nature of a chelate ring. The minor effect of a phenyl group is most likely due to the fact that the phenyl ring is twisted through a torsional angle of 70” from the mean plane of the metal-chelate ring as observed by X-ray crystallography of the copper chelate.‘*
1
1
I
I
I
N/nr Spectra. The nmr signal for the methyl protons in the metal complexes demonstrates downfield shift in general from the corresponding signa! for the proton complex as shown in Table V. Similar nmr data for the 2,4-pentanedione complexes of beryllium, aluminum and palladium have been reported recently by lunge et aLi Three effects seem to be responsible for this trend: reduction of electron density at the methyl magnetic anisotropy caused by x-electron groups, current in a chelate ring and an additional magnetic anisotropy due to a benzene ring as a result of metalcoordination. The third effect needs some explanation. The methyl groups of a metal complex may be exposed to a remote magnetic shielding effect caused by a benzene ring of another ligand in the same molecule. Table V. Proton 2,4-pentanediones;
chemical shifts of the metal complexes Ccl, solution, 40-41’C
Ligand
Metal
CH,
2,4-Pentanedione
H Be” Pd” Al”1 Co”’
2.00 2.04 2.02 1.93 2.12
--
3-Phenyl2,4-pentanedione
H Be” Pd’* Al”1 Co”’
1.83 1.87 1.77 1.87 2.00
-
7.30 7.31 7.29 7.29 7.29
H Be” Pdlr AllI*
2.01 2.06 2.04 2.03 2.20
3.61 3.72 3.72 3.67 3.78
7.16 7.22 7.25 7.17 7.16
i
(1) 1
(2) 3-Benzyl2,4-pentanedione
QIII
(3)
(4)
(5)
(6)
(7) 7M)
600
500
300
WAVE NdMBER
7.00
KM-‘I
Figure 4. Observed vibrational frequencies of various iron complexes of 3-substituted 2.4-pentanediones in the 700-200 cm ’ region: 1, hydrogen; 2, phenyl; 3, p-bromophenyl; 4, pcyanophenyl; 5, p-nitrophenyl; 6, benzyl; 7, p-nitrobenzyl.
lnorganica Chimica Acfa I 2:2
j June, 1968
of
However. the extent of this effect may vary as the steric configuration of a metal complex changes; almost or tetrahedral conegligible with square planar ordination, but some influence with octahedral COordination. In spite of the above presentation of various effects which may cause the shift of proton signals, it is rather difficult to estimate the contribution of each effect at the moment. The nmr signals for the benzene-protons in the 3-phenyl-2,4-pentanedione and the 3-benzyl-2,4pentanedione complexes do not shift significantly from those in benzene (7.27 ppm) and in toluene (7.10 ppm), respectively. (18) 1.
J-
W.
Carmichael,
Jr.,
L.
Chem. Phys., 43, 3959 (1965). (19) H. IUWe, H. MUSO, and
793 (1968).
K. U.
Steinrauf, I.
Zahorsky,
and
R.
Chem.
L.
Belford.
Rer.,
101,
Infrared and Nuclear Magnetic Resonance Studies on Metal Complexes of 3-Substituted
2,+Pentanediones
Yukito Murakami,* Katsuyuki Nakamura, Hiroaki Uchida, and Yukiko Kanaoka Received
April 27,
In an effort to understand the nature of chelate rings of 2,4-pentanedione complexes, infrared and nmr spectra of some divalent and trivalent metal complexes of 3-substituted 2,4_pentanediones, where phenyl, pbromophenyl, p-cyanophenyl, p-nitrophenyl, benzyl and p-nitrobenzyl were these substituents, have been investigated. The introduction of any of these substituents to the 2,4-pentanedione complexes resulted in the lower frequency shift of C=O stretching vibration and the higher frequency shift of M-O stretching, Thus, these substituents tend to increase the strength of coordinate bond, and the relative strength of -M-O bonds follows the sequence: Cut’ < Pd”; Fe”r < Crr” < Cont. Three structural formula have been put forward to elucidate the nature of chelate rings of various metal complexes. The relatively minor effect of a phenyl group, which is comparable with a benzyl group, is probably due to the twist of a phenyl ring from the plane of a chelate ring. Nmr spectra have not provided much useful information.
1968
infrared study has been extended to the far infrared region, where the metal-ligand stretching vibration is expected to appear, and the normal coordinate analysis ‘JO has been shown even more efficient in cooperation with deuterium9,” and oxygen (**O)” isotope techniques. In connection with the previous study on the electronic spectra of some trivalent metal complexes of 3-phenyl- and 3-benzyl-2,4-pentanedione,12 the present investigation deals with the infrared spectra of metal complexes of 2,4-pentanediones (I), which carry phenyl,
R = GH.T,
p-BGHT,
p-CNGHi;
pNOGHt,
CJ-LCHF,
p-NO,CbH,CHr Introduction
M = Ben, Cuit, Pd”; Al”‘, CrI*r, Fe”‘, Co’11
In recent years, the infrared spectral method has been much applied to the metal chelate systems of 2,4pentanediones for the elucidation of the nature of The these chelate rings and coordinate bonds.‘-’ normal coordinate treatment of these metal chelates has been much successful in the analysis of their vibrational spectra and become reliable basis for the In adassignment of those characteristic bands.‘,’ dition, the isotope techniques, where 2,4-pentanedione complexes labeled with deuterium, oxygen (“0) and carbon (13C) isotopes were used, have provided much valuable information for the assignments of absorption bands in the NaCl region and for the valence-tautomerism involved in the chelate rings.7,8 Meanwhile, the
p-substituted phenyl, benzyl or p-substituted benzyl group at the 3-position, in the 4000-200 cm-’ region. The assignments of vibrational bands have been performed by referring to the results of normal coordinate calculation on the 2,4-pentanedione complexes and to some empirical data. ‘J*‘~ The nmr measurements have also been added to understand the nature of these chelate rings.
Experimental Section Preparation of Metal Complexes.
procedures (*) To whom all the correspondence should’ be addressed. (1) K. Nakamoto and A. E. Martell, 1. Chem. Phys., 32, 588 (1960). (2) K. Nakamoto, P. 1. McCarthy, A. Ruby, and A. E. Martell, I. Am. Chem. Sot., 83; 1066 (1961). (3) K. Nakamoto, P. 1. McCarthy, and A. E. Martell, I. Am. Chem. Sot., 83, 1272 (1961). (4) K. Nakamoto. Y. Morimoto, and A. E. Martell. I. Phys. Chem.. 66, 346 (1962). (5) K. E. Lawson, Specfrochim. Acta, 17, 248 (1961). (6) R. D. Gillard, H. G. Silver, and I. L. Wood, Spectrochim. Ado, 20. 63 (1964). (7) H. Musso and H. lunge, J‘elrahedron L&f., 4003 (1966). (8) H. Musso and H. Junge, Chem. Ber., 101, 801 (1968).
The preparative for the divalent and trivalent metal com-
(9) G. T. Behnke and K. Naknmoto, Jnorg. Chem.. 6, 433 (1967). (IO) M. Mikami, 1. Nakagawa, and T. Shimanouchi. Specfrochim. Acfu, 23A. 1037 (1967). (1 I) S. Pinchas. B. L. Silufz, and I. Laulicht, 1. Chem. Phys.. 46. 1506 (1967). (12) Y. Murakami and K. Nakamura. Buff. Chcm. Sot. lapan, 39, 901 (1966). (13) See. e.g., N. B. Colthup, L. H. Daly. and S. E. Wiberley. .Introduction to Infrared and Raman Spectroscopy*, Academic Press, New York (1964).
Murakami, Nakamura, Uchida, Kanaoka 1 Infrared and Nuclear Resonance Studies on Metal Complexes
134 Table I. Analyses of the metal complexes of 3-substituted 2,4- pentanediones 3-Substituent
C
Complex Pound
Calcd.
Analyses, % H Calcd. Pound
N Found -
Calcd. -
Phenyl
73.21 63.63 58.18 71.40 68.59 67.74 66.03
73.52 63.83 57.85 71.73 68.62 68.17 67.81
6.32 5.49 5.08 6.21 5.78 5.85 5.68
6.17 5.36 4.85 6.02 5.76 5.72 5.69
p-Bromophenyl
51.26 46.54 48.42 48.23 48.01
51.09 46.22 48.67 48.45 48.26
4.02 3.81 3.76 3.98 3.85
3.90 3.53 3.71 3.70 3.68
-
-
pcyanophenyl
70.21 62.13 65.88
70.40 62.13 65.86
5.37 4.36 5.02
4.92 4.35 4.61
6.84 6.41 6.06
6.84 6.04 6.40
p-Nitrophenyl
60.82 52.19 56.24 55.00
58.79 52.43 55.32 55.08
4.73 4.17 4.55 4.41
4.49 4.00 4.22 4.20
5.90 5.83 4.76 5.64
6.23 5.56 4.86 5.84
Benzyl
74.10 65.19 59.50 72.61 70.06 69.38 68.74
74.40 65.22 59.45 72.71 69.78 69.34 69.00
6.79 6.13 5.65 6.75 6.35 6.58 6.44
6.76 5.93 5.41 6.61 6.34 6.30 6.27
-
-
p_Nitrobenzyl
60.34 53.97 57.48 60.37
60.37 54.19 57.29 57.00
5.29 4.69 4.86 4.95
5.06 4.55 4.81 4.78
5.92 5.19 5.91 4.97
5.87 5.27 5.57 5.54
plexes of 3-substituted 2,4_pentanediones were essentially similar to those employed for the corresponding 2,4-pentanedione complexes. A part of these experimental results have been described previously.14 The results of microanalyses” are listed in Table I. Spectral Measurements. Infrared spectra in the region 4000-650 cm-’ were recorded on a Koken Model DS-30 1 spectrometer equipped with sodium chloride optics. The KBr disk method was employed for the most complexes in this region, whereas the nujol mull technique was adopted for the metal complexes of 3-(p-nitrobenzyl)2,4-pentanedione. A Hitachi Model EPI-L grating spectrophotometer was used to obtain the spectra in the 700-200 cm-’ region. For these spectra, all the samples were measured as nujol mulls sandwiched between polyethylene film. Calibration of frequency readings was made with polystyrene film, and water vapor as well as carbon dioxide in the air. Nmr spectra were recorded on a Varian A-60 spectrometer. Chemical shifts are reported in ppm from internal tetramethylsilane (TMS) and data are good to 0.5 cps. The spectra were calibrated by the use of chloroform signal (436 cps from TMS) as the secondary reference.
(14) K. Nakamura and Y. Murakami, Tech. (Kyushu Daigaku Kogaku Shuho), 38, 329 (1965). (15) The analyses of all the metal complexes Microanalysis Center of Kyushu University.
Inorganica Chimica Acta I 2:2
i june,
Reports were
1968
Kyushu Univ.
performed
at the
Results and Discussion Infrared Spectra. The region 4000 to 2250 cm-’ has been excluded from this discussion, since no significant absorptions are seen except the C-H stretching bands. Then, the infrared frequency range treated in this work is divided into six regions for the sake of discussion: 2250-1300 cm-‘, 1300-1050 cm-‘, 1050-1000 -* 950-900 cm-‘, 900-700 cm-’ and 700-200 cm-‘. Fte ‘characteristic absorption bands along with the most plausible assignments for the 3-phenyl-2,4pentanedione complexes are summarized in Table II, while absorption peaks assigned to the C=O stretching and the M-O stretching modes for all the metal complexes treated in this work are listed in Tables III and IV, respectively. The observed bands for the metal complexes of 3-phenyl-2,4-pentanedione are shown schematically in Figure 1 in the 1600-700 cm-’ region for the sake of correlation. Figures 2 and 3 illustrate the far infrared spectra for the same metal complexes in the 700-200 cm-’ region. The 2250-1300 cm-’ region involves several distinct bands which are assigned to the C-O stretching, the C=C stretching, the CHs deformation and the C-CBS stretching modes in addition to the benzene-ring stretching vibrations. Although the assignment of vibrational bands for the 2,4-pentanedione complexes has been made on the purely empirical basis,r6,i7 (16) 1. Lecomte. Discussions Faraday Sot., 9, 125 (1950). (17) R. Mecke and E. Funck, Z. Elekfrochem., 60, 1124 (1956).
135 Table II.
Infrared
absorption
bands of the metal complexes
of 3-phenyl-2,4-pentanedione
Ben
Cu”
Pd”
Al”1
CrIn
Fc’ll
Co”
1582 vs 1459 s
1576 vs 1490 s 1454 s
1554 vs 1490 w
1595 vs 1462 s
I575 vs 1490 w 1453 m
1573 vs 1490 w 1452 s
1439 sh
1424 s
1439 m
1439 m
1437 s
1383 s 1369 s 1335 m 1250 w 1179vw 1157 VW 1073 w 1021 s
1381 s 1381 s 1315 s 1175w 1154w 1075 w
1428 m 1420 m 1360 m 1335 s 1304 m 1241 w 1176~ 1155w 1072 w 1036 w 1014 m 1003 w 976 w 929 w
1570 vs 1490 w 1477 w 1451 m 1423m t
1396 s 1372 s 1330 m 1155vw 1075 w
1373 m 1349 s 1318 m 1178~~ 1157vw 1074 w
1363 s 1346 s 1318 s 1173 w 1158~ 1074 w
1038 w 1019 m
1036 w 1014 m
1035 w 1012 m
983 w 932 m 749 m 705 m 1042 s 856 m 845 s 684 w 660 s 617~ 553 s
499 w 462 w 449s 440s 430 w 369 w 283 w 245 m, br s = strong,
Table
Ill.
1035 w 1012 m 979 930 921 764 710
w w w s s
768 w 721 w 706 w
983 w 930 w
979 w 927 m
770 w 707 m
778 sh 766 w 709 m
970 926 916 769 764 707
Assignments
benzene
693 s 618w 556 m
683 m 665 w 625 m 608 s 560 vw 533 m
684 661 634 617 538
470 457 426 393
474 458 442 408
508 492 455 448 432 425
466 s, br 439 s 397 m
s s m s
305 w 271 m
313 m 227 s
m = medium,
The C=O
s m w w
bands
350 s 291 w 247 w
377 m 313w 239 w
and w = weak intensity;
stretching
s s m m sh m
(cm-‘)
m VW s s w
v = very,
sh = shoulder,
for the metal complexes
ring
1360 m 1349 m 1315 m
C=O str CH, sym def C-C + C-CH,
1155 w 1074 w 1225 w 1 1037 w
benzene in-plane
str
C-H
CH, deg rock
1018 1003 w 1 980 w 929 w \ I 765 w
..... C-CH,
+ C-O
ring def + M-O C-CHI bend +M-0 str
680 w 655 w 618w 598 s 588 s 557 VW 522 w 451 s 437 s 425 s 415 sh 390 m
688 s 650 s 617~ 555 m
473 s 421 w
M-O
str
315 s 234 m, br 220 sh
385 w 361 m
M-O
str
of 3-substituted
2,4-pentanediones
Ligand
Ben
cun
Pd”
Al’”
Cr”’
Fe”’
Co”’
1582 1583 1582 1582 1578 1585
1576 1574 1568 1575 1569 1571
1554 -
1595 1595 -
1575 1573 1568 1583
1573 1570 1571 1570 1570 1566
1570 1570
Abbreviations are made for ligands: PAA, 3-phenyl-2,4-pentanedione; NPAA, 3-(p-nitrophenyl)-2,4-pentanedione; cyanophenyl)-2,4-pentanedione; benzyl)-2,4_pentanedione.
Nakamoto et al.‘~*first applied the normal coordinate treatment to these metal complexes and the strong band at higher frequency side in the 1600-l 500 cm-’ region was assigned to the C=C stretching mode. Recently, however, Behnke and Nakamoto has made the normal coordinate analysis of dichloro(2,4-pentanediono)platinate( II) by introducing a modified Urey-Bradley force field? In cooperation with the deuterium isotope effect on vibrational spectra of the 2,4-pentanedioneplatinum complexes, they have proposed to interchange the band assignments for the C-0 and the CzC Murakami,
Nakamura,
Vchida,
Kanaoka
str
and br = broad.
PAA BrPAA CPAA NPAA BAA NBAA
1556 -
str
benzene C-H out-of-plane
704 m ) I I
682 m 664 w 635 s 618 w 541 m
str
CH, deg def
I
w w m sh w m
(+C-C)
C=O
1570 1567 -
BrPAA, 3-(p-bromophenyl)-2,4-pentanedione; CPAA, 3-(pBAA, 3-benzyl-2,4-pentanedione; NBAA, 3-(p-nitro-
stretching mode. This result has been supported by investigation of the vibrational band-shift in the infrared spectra of the ‘80-labeled 2-4-pentanedione complexes of chromium( I I I) and manganese( I II),” and also of the *H-, “0-, and “C-labeled 2,4-pentanedione complexes of copper( I I)? Another interchange of the band assignments made by Nakamoto and his coworker’ is for the CH3 degenerate deformation and the lower frequency C-0 stretching mode, and this has been supported by the infrared study of the corresponding siotope-labeled complexes as reported by Musso and 1 Infrared
and Nuclear
Resonance
Studies
on Metal
Complexes
136 Table
IV.
The M-O
Ligand
stretching
Be”
PAA
1042 856 845
BrPAA
1034 844 482 1037 853 489 1031 855 505 859 508 1015 863 507
bands
(cm-‘) for the metal complexes
CU”
Pd”
of 3substituted
Al”’
2,4-pentanediones
Cr”*
Fetn
Co”’
470 313
474 305
508
466 350
451 315
473 385
463 298
-
-
470 355
453 312
478 386
467 314
-
-
-
452
468 313
-
-
-
453 311
474 384
465 292 465 310
478 297 -
505
469 372 467 363
452 318 456 317
479 391 -
499
CPAA NPAA BAA NBAA
Abbreviations
for ligands
are decribed
-
312
-
in Table III.
Ben Cu” Pd”
Crrn Feru
Figure 1. complexes
Observed vibrational frequencies of various metal of 3-phenyl-2,4-pentanedione in the 1600-700 cm- ’
.-:
700
600 WAVE
&I
NJMBER
&I ’ &I ’
I 200
ICH-‘)
Figure 3. Far infrared spectra of trivalent metal complexes 3-phenyI2,4_pentanedione (nujol mull method).
WAVE NlM8ER
KM-‘)
Figure 2. Far infrared spectra of divalent metal complexes of .3-phenyI2,Cpentanedione (nujol mull method). Inorganica
Chimica
Acta
1 2:2
1 lune, 1968
of
Junge.’ The two strong bands in the 1600-1500 cm-’ region observed for 2,4pentanedione complexes seem to be merged into one strong band in the present chelate ,,“.A +l.:* &,,..A :- (133LUUSU ,.^,...... ,.A e,. ,..:-,..:1.. ,I.., n.rn*--0 Llll” 3J3LG1113, L‘LLD “a,lU ID L” L, “F; ~“““cl”‘y uue to the Cc0 stretching vibration. When 3-(p-nitrophenyl)-2,4-pentanedione and 3-(p-nitrobenzyl)2,4pentanedione are used as ligands, the asymmetric and the symmetric NO* vibrational mode appear evidently at around 1515 cm-’ and 1350 cm-‘, respectively. On the other hand, in the case that 3-(p-cyanophenyl)-2,4pentanedione is a ligand molecule, the band near 2220 cm-’ is due to the CN stretching mode.
137
In the regions 1300-1050 cm-’ and 900-700 cm-‘, there are observed several absorption bands which are characr=rist:c of benzene C-H in-plane bending and out-of-l lane wagging vibrations, respectively. In addition to these vibrational modes, the aromatic C-Br stretching mode in the 3-(p-bromophenyl)-2,Cpentanedione complexes is expected to appear in the former region although the definite position is not certain in the present work. On the other hand, the latter region involves the C-N stretching mode for the nitrocompouds; i.e., in the 854-860 cm-’ range. The CH, rocking mode in the 1050-1000 cm-’ region resulted in a larger number of bands in most cases by introducing a substituent at the 3-position of 2,4pentanedione. In the lowest frequency region in this work, 700-200 cm-‘, the vibrational spectra of all the metal complexes became much more complicated than the corresponding 2,4-pentanedione complexes, due to .the presence of an extra substituent. However, a strong band in the 450500 cm-’ range and another band of various intensity in the 300-400 cm-’ range are assigned to the metaloxygen stretching coupled with other vibrational modes for the copper, palladium, chromium, iron and cobalt complexes. Behnke and Nakamotog has assigned the lower frequency band near 300 cm-’ to the C-CHJ bending mode in their study of the 2,4-pentanedioneplatinum(I1) complexes. On the other hand, Mikami, Nakagawa and Shimanouchi*” have observed that the lower frequency band is most metal-sensitive. The latter assignment is preferred in the present work as described above. The spectra of all the beryllium complexes are markedly different from those of the corresponding complexes of other metals in three regions; 1050-1000 cm-‘, 900-700 cm-’ and 700-200 cm-‘. In conformity with the previous assignments made by Nakamoto et aL3 for the 2,4-pentanedione complex, the strong bands appearing near 1030 cm-’ and in 860-830 cm-‘, and the medium band near 500 cm-’ are assigned to the Be-O stretching mode coupled slightly with other vibrational components. It is reasonable to expect that the beryllium complexes demonstrate their metalsensitive bands in relatively higher frequency region than other metal complexes, due to the light mass of beryllium atom and the very short Be-O distance. Judging from the relative positions of the C-O bands in the 1600-1500 cm-’ range among the metal complexes of each 3-substituted 2,4-pentanedione, delocalization of ligand z-electrons in a chelate ring tends to increase in the_following sequence: for divalent
metals,
Be < Cu< Pd
for trivalent metals, Al< Cr < Fe <, Co. complex demonstrates the C=O vibrational mode at the lowest frequency in the above range
The palladium among ligand, among
the metal complexes of each corresponding The band-shift of this mode is seen to be minor other transition metal complexes. The C--O bands in the beryllium and aluminum complexes are observed at relatively higher frequency than the other metal complexes as shown in Table 111 and the delocalization of n-electrons in a chelate ring of the Murakami,
Nakamura,
Uchida,
Konaoka
beryllium co.nplexes seems in general to be slightly greater than the corresponding aluminum complexes. Another C-O stretching mode appearing in the 14001360 cm-’ range varies in a manner parallel to the higher frequency mode among the metal complexes of each 3substituted 2,4-pentanedione. The metal-oxygen stretching modes appearing in the 700-200 cm-’ region may directly provide the information on the relative strength of coordinate bonds, and the force constant of the oxygen-metal bonds in the transition metal complexes of each ligand species follows the sequence: for divalent metals, Cu< Pd for trivalent
metals,
Fe
This is in agreement with the results obtained by the two research groups on the 2,4-pentanedione complexes. 2,3.‘oThe coordinate bond of a palladium complex seems to be strongest among those of the transition metal complexes treated in this work. In order to elucidate the electronic structure of various metal complexes investigated in this work in conformity with the electronic spectral dataI as well as the present results, three structural forms are put forward below: i.e., strong (II), weakly g-bonded (III) and strongly a-bonded (IV) chelate rings. In strong
chelates, ligand n-electrons as well as d-electrons of the metal tend to be well delocalized in a whole chelate ring. The palladium chelates are typical of this structure. In weakly c-bonded chelates, ligand n-electrons are well extended to the coordinate bonds and tend) to be delocalized in a whole chelate ring. Nevertheless, weak a-bonding provides relatively weaker coordinate bonds as a whole. This seems to be the case for the iron complexes. As understood by investigating the X+X* transitionI and the carbonyl stretching mode for the first transition metal complexes of 3-substituted 2,4pentanediones, x-electrons are well delocalized in a chelate ring.and d-orbitals of a trivalent metal atom are involved in such a x-system. This n-electronic interaction in the iron complexes is relatively large among the trivalent metal complexes of present concern and greater than that in the corresponding chromium complexes. Nevertheless, the coordinate bonds involved in these iron complexes are weaker than those in the corresponding chromium complexes as the M-O stretching vibrations appearing in the far infrared The above inconsistency can be region indicate. ascribed to the weaker c-bond character of the coMeanwhile, ordinate bonds in the iron complexes. ligand x-clcctrons in the beryllium and aluminum complexes most likely tend to be delocalized only in a ligand part as judged from the C=O vibrational modes , Injrared
and Nucleur
Kesonunce
Studies
on Me&d Complexes
138 and the electronic 7c+rc* transitions” of these metal The third structural form (IV) may be complexes. assigned to these non-transition metal complexes. The introduction of either phenyl or benzyl derivative at the 3-position of 2,4-pentanedione resulted in the lower frequency shift of C-O stretching vibration and the higher frequency shift of M-O stretching vibration. Thus, the substituents employed in this work are in favor of forming a stronger chelate ring. However, the characteristic bands cited here do not vary significantly among the various 3-substituted 2,4pentanedione complexes of each metal species. The variation of far infrared spectra due to the change of ligand is shown in Figure 4 for the iron complexes. Thus, the effect of the substitution with a phenyl group on the electronic structure of a chelate ring is almost the same as that with a benzyl group; in addition, psubstituents of these groups do not cause much change on the nature of a chelate ring. The minor effect of a phenyl group is most likely due to the fact that the phenyl ring is twisted through a torsional angle of 70” from the mean plane of the metal-chelate ring as observed by X-ray crystallography of the copper chelate.‘*
1
1
I
I
I
N/nr Spectra. The nmr signal for the methyl protons in the metal complexes demonstrates downfield shift in general from the corresponding signa! for the proton complex as shown in Table V. Similar nmr data for the 2,4-pentanedione complexes of beryllium, aluminum and palladium have been reported recently by lunge et aLi Three effects seem to be responsible for this trend: reduction of electron density at the methyl magnetic anisotropy caused by x-electron groups, current in a chelate ring and an additional magnetic anisotropy due to a benzene ring as a result of metalcoordination. The third effect needs some explanation. The methyl groups of a metal complex may be exposed to a remote magnetic shielding effect caused by a benzene ring of another ligand in the same molecule. Table V. Proton 2,4-pentanediones;
chemical shifts of the metal complexes Ccl, solution, 40-41’C
Ligand
Metal
CH,
2,4-Pentanedione
H Be” Pd” Al”1 Co”’
2.00 2.04 2.02 1.93 2.12
--
3-Phenyl2,4-pentanedione
H Be” Pd’* Al”1 Co”’
1.83 1.87 1.77 1.87 2.00
-
7.30 7.31 7.29 7.29 7.29
H Be” Pdlr AllI*
2.01 2.06 2.04 2.03 2.20
3.61 3.72 3.72 3.67 3.78
7.16 7.22 7.25 7.17 7.16
i
(1) 1
(2) 3-Benzyl2,4-pentanedione
QIII
(3)
(4)
(5)
(6)
(7) 7M)
600
500
300
WAVE NdMBER
7.00
KM-‘I
Figure 4. Observed vibrational frequencies of various iron complexes of 3-substituted 2.4-pentanediones in the 700-200 cm ’ region: 1, hydrogen; 2, phenyl; 3, p-bromophenyl; 4, pcyanophenyl; 5, p-nitrophenyl; 6, benzyl; 7, p-nitrobenzyl.
lnorganica Chimica Acfa I 2:2
j June, 1968
of
However. the extent of this effect may vary as the steric configuration of a metal complex changes; almost or tetrahedral conegligible with square planar ordination, but some influence with octahedral COordination. In spite of the above presentation of various effects which may cause the shift of proton signals, it is rather difficult to estimate the contribution of each effect at the moment. The nmr signals for the benzene-protons in the 3-phenyl-2,4-pentanedione and the 3-benzyl-2,4pentanedione complexes do not shift significantly from those in benzene (7.27 ppm) and in toluene (7.10 ppm), respectively. (18) 1.
J-
W.
Carmichael,
Jr.,
L.
Chem. Phys., 43, 3959 (1965). (19) H. IUWe, H. MUSO, and
793 (1968).
K. U.
Steinrauf, I.
Zahorsky,
and
R.
Chem.
L.
Belford.
Rer.,
101,